A major problem with above-ground liquid storage tanks, particularly those that contain liquids which can adversely contaminate the surrounding grounds, is to be able to determine if the tank leaks. This problem becomes greater in large tanks, greater than about 20 feet in diameter, since relatively large leaks will cause only a small change in the level of the liquid in such tanks. Also, in above-ground tanks, changes in the ambient, such as temperature and wind, can affect the level of the liquid in the tank.
Heretofore, the best available technique for determining leaks for above-ground tanks was to put a six foot head of water in the tank and use visual detection of liquid level variations in a sight glass outside the tank to measure possible leaks. This technique has the disadvantage that the accuracy of the visual detection is limited to about 1/16 inch variation in the tank. This means that for a 100 foot diameter tank there must be a leak of at least 300 gallons and for a 50 foot diameter tank there must be a leak of about 100 gallons to obtain a detection of the leak. Also, changes in temperature, which can cause the liquid in the tank to expand, significantly affects the measurement as well as wind effects and sight glass meniscus resolution.
Another technique which has been considered is to place a pressure transducer adjacent the bottom of the tank which measures the pressure of the liquid at the bottom of the tank. This technique is not affected by changes in temperature since although the volume of the liquid may change due to expansion of the liquid, its weight does not change. This technique has the problem that it requires a transducer which can not only measure the heavy weight of the liquid in the tank, but which can also measure slight differences in the weight. Such a transducer is either not readily available or is expensive.
Swiss Patent No. 607,011 discloses a checking device for leakage from large liquid reservoirs and tanks which utilizes a pressure sensitive detector having two chambers separated by a diaphragm. One chamber of the detector is connected to the reservoir and the other to a reference tube. The diaphragm of the pressure sensitive detector can be moved toward a contact member to close an electrical signalling circuit in the event that a leak develops. However, the arrangement disclosed in Swiss Patent No. 607,011 would not be expected to have the accuracy required to assure tank integrity in light of today's environmental concerns and regulations.
Therefore, it would be desirable to have a detector for accurately detecting small leaks in large above-ground tanks which is not affected by changes in ambient conditions and which is relatively inexpensive.